The present invention relates to vehicle steering control systems and apparatus for limiting the steering radius of a vehicle at elevated speeds to provide improved maneuverability, prevent damage to the steering mechanism, and prevent damage to surfaces over which such vehicles travel, such as, turf. More particularly, the present invention relates to a novel hydraulic steering cylinder for controllably limiting movement of the steering linkage, thereby limiting turning radius.
Numerous wheeled vehicles employ a steering mechanism connected to steered wheels so that the wheels can be operated in a synchronous manner to steer the vehicle. The steering mechanism includes a steering linkage which is constructed to provide a common turning center for the wheels. Often a hydraulic powered steering system is incorporated with the steering mechanism to improve the ease of operation of the steering linkage. Hydraulic powered steering systems are commonly used with a wide variety of motorized vehicles.
A hydraulic cylinder is incorporated in the powered steering system to controllably operate the steering linkage. The hydraulic cylinder houses a single piston which is hydraulically moved in order to drive an attached shaft. The hydraulic cylinder has a stroke length defined by the movement range of the piston in the cylinder. The stroke length in conjunction with the linkage structure determines the range of turning radii achievable using the particular steering mechanism. In other words, the longer stroke length provides a greater turning radius range and thus can achieve a shorter turning radius, whereas the shorter stroke length provides a smaller turning radius range thereby producing a longer turning radius.
Several problems arise with such a hydraulic powered steering mechanism, the foremost problem being the lack of ability to controllably limit turning radius at elevated speeds. An example of vehicles which would benefit from a solution to the turning radius problem include golf course maintenance vehicles, tractors, turf management vehicle of various types, golf carts and small industrial vehicles. These vehicle typically have short wheel bases, are capable of moving quickly, and often are capable of making very tight turns. As a result of the lack of turning radius control in these vehicles the steering mechanism may be damaged when making tight turns at high speeds, as well as, the surfaces over which such vehicles travel, such as turf.
Using a golf course turf management vehicle as an example, the turning radius problems discussed above can result in damage to the vehicle itself, the turf and fairways over which the vehicle travels and make routine tasks less efficient. The turf surfaces of golf courses are very expensive works of landscaping design which require continuous maintenance and protection to preserve desired playing conditions. Turf management vehicles are used to carry out the various tasks which are involved in maintaining and protecting the turf surfaces. The tasks include, among others, mowing, fertilizing, aerating, delivering supplies to various areas of the course, and transporting people.
Often, turf management vehicles are required to travel relatively long distances through the golf course to carry out a task, and as such, must be capable of traveling at increased speeds in the interest of saving time. The turf surface, however, must not be damaged by the vehicles. If the turning radius of a vehicle is not limited while traveling at high speeds, a sharp turn can cause the vehicle to slide or skid on the turf thereby tearing or digging into and damaging the turf. Additionally, at low speeds the vehicles need to be able to make tight turning radius precision maneuvers such as hair pin turns while fertilizing, aerating, mowing, etc. and parking.
Currently available turning radius limiting devices do not accommodate all of the needs of such turf management vehicles. Typically, the available turning radius devices allow tight turning, but in doing so, reduce vehicle speed so that the turf is not damaged when making a tight turn. Alternatively, the available turning radius devices allow higher speeds, but in doing so, correspondingly increase the turning radius to prevent turf damage at the higher speeds. More specifically, one form of turning radius limiting attempts to limit the stroke length of the control or hydraulic cylinder thereby limiting the turning radius range of the steering mechanism. The preferred limit of the turning radius range is defined by the tightest radius which can be achieved at maximum speed with out damaging the steering mechanism or the travel surface. By limiting the turning radius range the vehicle cannot be controlled into a tight turning radius which might cause damage to the steering mechanism or the turf.
Devices which attempt to overcome the above-noted problems include mechanical stops attached to the steering linkage to physically obstruct movement of linkage components. By obstructing the movement or operation of the components of the steering mechanism, the mechanical stops limit operation of the steering mechanism to a predetermined turning radius corresponding to the relationship between the blocks and the component thus blocked. While such mechanical stops provide a limiting effect on the turning radius, they also create problems by restricting turning radius for all speeds. Another problem with the mechanical stops is that they are exposed to the elements and therefore are subject to wear and possible damage. Additionally, mechanical stops, when damaged, may be overridden by forcing the obstructed component past the block or by disassembly and removal of the block from the steering linkage.
Specific examples of the devices generally described above are shown in U.S. Pat. No. 5,022,480 to Inagaki et al. and in U.S. Pat. No. 4,109,748 to Evans. The Inagaki '480 reference shows a steering rack which operates the steering linkage. A mechanical linkage is controllably positioned for mechanically limiting movement of the rack to limit the turning radius of the linkage. This device, however, is dependent upon proper engagement of the mechanical linkage with the rack and as such is subject to failure if the mechanical device does not properly engage the rack. Additionally, if the rack is not properly positioned in relation to the mechanical limiting device, the limiting device will not properly limit the turning radius and may actually interfere with the safe operation of the rack.
The device as shown in Evans '748 employs a pair of mechanical stops which are independently controllably engagable with the left and right steering knuckles to mechanically limit the turning radius of the steering linkage. The limiters as shown in Evans '748, introduce other problems by including additional independent operating components in the steering system. The limiters add to the overall cost and maintenance of the steering system and present another potential failure point to the steering system.
Control or limiting of the turning radius of a vehicle, as described above, has helped, to some degree, prevent damage to vehicles and turf. Prior art limiting devices create other problems by restricting turning radius over the entire speed range of the vehicle and thus greatly affecting maneuverability at low speeds. This is a problem because the turning radius generally does not serve any function at sufficiently low speeds. In other words, the limiting function is primarily only necessary within a high speed range where the combination of variables, such as a tight turning radius and sufficiently high speed, could culminate in damage to the vehicle or turf. As such, the prior art turning radius limiting devices are bothersome, inefficient, and unnecessary at sufficiently low speeds. This problem is exacerbated when one considers the numerous steering functions (i.e. precision maneuvering, hairpin turning such as when mowing, fertilizing, aerating, parking, etc.) which are executed at low speeds. If a tight turning radius is prohibited, such precision steering functions become very time consuming and perhaps impossible.